I subscribe to the theory that everyone is born with a subconscious knowledge of their sex. If that neurological or 'brain sex' aligns well with your physiological sex, you won't experience gender dysphoria. One's subconscious sexual identity (I'm not talking sexuality here) can lie dormant for years. I think my gender dysphoria existed from an early age, but I didn't recognize it, and rationalized its effects to other causes. Eventually, however, it broke through to the conscious level.
There are many factors which accelerate or impede this change. For much of my life the vocabulary and understanding of gender identity didn't even exist outside a tiny sliver of the medical establishment. Social gender conditioning and enforcement overwhelmed and inhibited entertaining ideas of cross gender identity. Even when I became consciously aware of my sexual identity and attributed the psychological distress I was experiencing to it, I continued to deny its importance and suppress its influence, but with no lasting effect. At each stage of my life the effects of GD became more intense. Coping mechanisms broke down and were replaced by still more effective means to evade the life threatening consequences of accepting the truth of who I really was.
So in my view, gender dysphoria is not something that strikes you like a cancer. The seeds of GD were planted before I was born. It was just a matter of time for it to grow, and make its presence known, and for me to finally accept it.